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I've read that one should use "ir a" when time of completion is certain. If the time is uncertain, one should use the future tense. This choice is not so clear-cut when asking a question. Take for example, asking "When are you coming?". You're expecting an exact time from the other party. But the other party may not know the exact time or whether he can come at all.

¿Cuándo vas a venir?

VS

¿Cuándo vendrás?

So which one do you use when one side wants a precise time while the other side cannot be guaranteed to provide an exact time?

In my experience speaking spanish with people from Mexico, El Salvado, Ecuador, Peru, and Columbia, it is also the same, voy a <verbo> and verbo en tenso futuro are the same, and can be used interchangeably.
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EricJul 18 '13 at 16:53

1

I'm mexican and I also use them indifferently, I would even use: ¿Cuándo vienes?
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NewbieJul 18 '13 at 17:43

Sorry about the mispelling. I got it mixed up with Latin. I did a quick search on "ir a versus future tense" on Google and all the articles/discussions say "ir a" is definitive, while future tense is uncertain. Here is just one of the many discussions.
–
JoJoJul 19 '13 at 3:55

It usually does but it can mean a future o sporadic action depending of the context´-"Voy a ir a México" -"enserio?, ¿Cúando vienes?. '-"I'm going to Mexico" -"Really?, When are you comming?"´
–
NewbieJul 22 '13 at 16:19